1. Field of the Invention
The present invention relates generally to the field of pressure controls and, more particularly, to the field of compact, direct mount pressure controls for use on machinery such as that used for refrigeration, air conditioning and ice generation. Still more particularly, the invention relates to a positive action switch, wherein the switch is thrown to a first position when an actuator is moved to a first position and is returned to its initial position when the actuating plunger moves to a second position different from the first position. Still more specifically, the present invention relates to adjustment of the set and return points, including simplified control of the lost motion aspect of the invention.
2. Description of the Prior Art
On refrigeration, air conditioning and ice generation equipment, a variety of pressure controls have been used in the past to cycle the compressor, cycle the condenser fan, prevent system overload, etc. In the past, these controls have generally fallen into two categories. In the first, highly accurate controls which had easily accessible trip points and reset points were used and were adapted to various applications and field conditions. These controls normally relied on extensive lever mechanisms and multiple springs to make adjustments. These components made them unsuitable for mounting directly to the equipment due to the high vibration levels usually associated with such equipment. This necessitated mounting the control remotely from the equipment and connecting the control with a hose or capillary. Breakage or leakage in the hose or capillary would result in a loss of refrigerant charge.
The second type of prior art controls have been direct mount controls, which were usually constructed in a rugged manner to take the compressor vibration. However, these controls were usually less accurate than the type described above. The majority of the direct mount controls have trip points and reset points which are factory set and are not field adjustable. Even on those controls where field adjustment is possible, it has been very difficult and usually requires that the control be removed from the equipment to make the adjustment. Also, in the latter case, it has been a typical practice to adjust the differential by varying the contact gap, which in turn severely limits the load which the switch can effectively carry.
One type of control which has been employed is the positive action switch or snap switch. Generally speaking, it consists of a flexible arm which is contacted by an actuating plunger to force the flexible arm past an "over center position." As soon as the flexible arm is forced past such position, the construction of the switch is such that the portion of the arm carrying electrical contacts will move from positive contact with a stationary contact on one side to contact with a stationary contact on its other side by a single positive movement known commonly as the "snap." See, for example, U.S. Pat. No. 2,789,173, issued Apr. 16, 1957.
When the plunger is retracted away from the flexible arm in such devices, the flexible arm again passes over its over center position, which causes the flexible arm to "snap" the contacts carried by it back to their first position. Thus, a basic characteristic of these devices was that the switching action will occur at substantially the same position of the plunger, regardless of whether the plunger is traveling in one direction or the other, for there is only a very small distance of a few thousandths of an inch between each side of the over center position. Previously, deficiencies noted for such switches were overcome by utilizing more than one positive action switch whenever it was desirable to have switching in a circuit occur at different positions of the device which actuates the switch plunger.
In U.S. Pat. No. 3,233,055, issued Feb. 1, 1966, a switch was described which overcame the latter problem by providing means in a single positive action switch for differential switching positions of the actuating plunger. Furthermore, in the device shown in this patent, the distance of actuating plunger travel between switch operation in one direction and switch operation in the opposite direction was adjustable, utilizing the positioning of two "abutments" in different areas of the switch to engage opposite sides of the switch blade, depending on which direction the plunger was traveling. Movement of the plunger was opposed by a resilient device such as a spring, so that when the plunger was retracted, the switching operation in the opposite direction would occur only when an abutment carried between the spring and the switch arm reached the surface of the switch arm, resulting in a lost motion approach to switch adjustment. In such device, instead of only a few thousandths of an inch existing between switch operation and switch reactivation, switching in the opposite direction would not occur until the plunger has been withdrawn to a position on the order of thirty thousandths to one hundred fifty thousandths (30/1000-150/1000) of an inch further away from the position which would normally cause the switch to operate. The device provided means not only for adjusting the initial switch point, but also the distance between the differential positionings of the actuating plunger.
During operation of the switch most recently referred to, actuation of the plunger would cause contact on the upper surface of the switch blade, forcing it down until the blade would snap to the depressed position. The free end of the blade would bring the contacts from one location to another, resulting in the desired switch actuation. Following switching, typically the passive plunger, located on the opposite side of the switch blade, would start to move but would not cause the switch to move to its initial position until it had moved upwardly by approximately one hundred thousandths (100/1000) of an inch. An abutment on the passive plunger would, at that point, approach the lower side of the blade, eventually contacting it and providing the force to bulge the blade upwardly and snap it to its elevated, original position. The differential distance between the actuation positions was adjusted by rotating threaded plugs with a screwdriver, such adjustment varying the amount of protrusion of a pin beyond the abutments and therefore controlling the distance between the respective abutments described in the patent.
With the device described in the last-mentioned patent, control has been a problem, as has been the establishment of the relative pressures set by the various adjustment mechanisms.
A pressure switch which overcomes the disadvantages of the prior art would be a significant advancement in the art.